May 2, 1997

Connecticut Fruit Growers Newsletter

May 2, 1997

In This Issue:

* Brown rot considerations

* Apple thinning

* Twilight meeting #2, May 20

Why Spray Stone Fruits During Bloom?

In years with little or no bud-kill before bloom, such as this year, blossom blight from the brown rot fungus (Monilinia) is not likely to result in excessive fruit thinning even if not controlled by fungicide sprays during bloom. Does this mean bloom-time fungicides are not worthwhile this year? No, they are probably still worthwhile, for peach at least, for the following reasons:

1) Infections that begin in open peach blossom often move on into the woody portion of the year-old shoot, where they remain active as cankers, producing spores for a year or more. Spores produced by infected flowers and woody tissue, throughout the current growing season, can infect injured green fruit, aborted fruitlets (buttons) on the tree, green fruit thinned after the pit hardening stage and left on the ground, and ripening fruit near harvest time. All of these subsequently infected tissues can produce additional spores during the current growing season, increasing the likelihood of fruit infections shortly before harvest.

2) Brown rot cankers at the base of infected blossoms are susceptible to invasion by another serious fungus, Leucostoma, producing what is commonly called Valsa canker, or perennial canker. Leucostoma cankers eventually destroy the infected limb or tree.

Stone fruit species other than peach and nectarine evidently do not produce brown rot cankers in woody tissues in the northeastern United States; and leucostoma, even though it can infect a rather wide host range, is reported to be an important problem (in the northeast) only in peach, nectarine, and sweet cherry. Considering these differences between peach and plums, one might wonder whether blossom blight of plums is really worth getting concerned about, assuming one will probably want to use a rigorous preharvest rot-prevention program whether or not fungicides were applied during bloom.

How Many Blossom Sprays for Brown Rot?

Dr. Wayne Wilcox, Cornell, Geneva, has observed 2-3 days of after-infection control of blossom blight when using Orbit, Indar, Elite, Funginex, rovral, or Ronilan. These materials penetrate treated flower tissues and provide protection for at least two days after treatment time. He has not tried to infect treated flowers beyond 2 days after the spray, so we can=t say just how long this protection lasts. Labels permit first 2 or 3 blossom blight sprays, depending on the particular fungicide.

Captan is not known to provide after-infection control of blossom blight, and is not systemic. Spray intervals of 3-4 days may be needed in wet weather where Captan is used to prevent brown rot infection during bloom. The same holds for sulfur. With Bravo, the label limits application to popcorn, full bloom, petal fall and shuck split.

Dr. Wilcox has determined that the young fruilets of all stone fruit species except sour cherry are wuite susceptible to brown rot infection during the first few weeks after petal fall, during wet weather. Such infections often do not grow until harvest time approaches, but then produce fruit rot in spite of preharvest fungicide sprays. Only Elite, captan, and sulfur are registered for brown rot control during the early post-bloom period, except that Bravo can be used at shuck split.

Because effective fungicides are also needed during the last couple of weeks throughout is on the tree, repeated use of one type of fungicide could result in resistant brown rot. Ideally, the fungicide used near harvest time should be of different type than that used in the bloom period. Rovral and Ronilan are a similar class of compound, and should be considered the same with respect to resistance. Orbit, Indar, Elite and Funginex are all sterol inhibitor chemicals, and therefore should be considered the same with respect to resistance. If a sterol inhibitor is used for blossom blight, a sterol inhibitor should not be used at the preharvest time unless or until Rovral or Ronilan has been used a couple of times during the preharvest and harvest period. Thus, only during long harvest seasons would you expect to use more than 2-3 spays of one type of these two classes of resistance-prone fungicides.

Apple Thinning

Although a caustic blossom thinning chemical, called Wilthin now has federal label, it is registered in only a few states for 1997. No application for registration in Connecticut has been filed, in it will not be available here for bloom-time. There are so many unanswered questions about use of Wilthin under eastern USA conditions, that any use this year would be very much on trial basis, anyway.

I consider Accel also to be for trial use. It is a week thinner, at label-permitted rates, and I am uncertain of the conditions under which it can produce seedless fruits that remain small and fail to drop, as seen in my trials last year.

That leaves us with NAA and Sevin as choices for the bulk of the thinning requirements. I guess NAA (Amid) is a choice, but it also is a rather week thinner. While Sevin has some real advantages (moderate thinner, does not stunt fruit growth, is unlikely to over-thin), but because it kills our current populations of mite predators (A. fallacus, Z. mali, Stethorus), I have avoided its use.

NAA is quite effective as a thinner when weather conditions are favorable but resulting fruit size is often less than one would expect for the crop load. Work by Terance Robinson, Cornell, showed that little or no size reduction occurred if NAA was applied at petal fall, as compared to 2 weeks after petal fall. Petal fall treatments were less effective in thinning, at comparable rates. Relatively higher rates are expected to be needed if one hopes to get a lot of thinning from petal fall NAA.

What follows are summary statements made by Dr. Ed Stover, Cornell, Hudson Valley Lab, in a handout at a talk to Hudson Valley Growers on April 2, 1997. Where he mentions use of combination sprays of NAA/Sevin or Accel/Sevin, it is my understanding that he is referring to use of Sevin at the rate of one-half pound active ingredient per 100 gallons, dilute spray equivalent; for example, one pound of Sevin 50WP per 100 gallons water.

Time of thinning

* in general earlier thinning will give larger fruit size at a comparable cropload

* use of multiple thinning treatments may give better control of cropping in difficult to thin blocks, with higher profitability and probably better return bloom

* blossom thinners offer several advantages: 1) will often work even when other thinners do not, 2) response is evident before petal fall treatments would be applied, 3) may significantly enhance return bloom in strongly alternate bearing varieties

* blossom thinner disadvantages: 1) little NY experience (WilThin is currently labeled in NY, suggested use at higher label rates, and include 1 pint Regulaid/100 gallons, apply at 60-80% bloom) (ATS, ammonium thiosulfate, not labeled as a thinner in NY, but may cause some thinning if applied as a nutrient at 60-80% bloom, typically used at 1% ATS), 2) some foliar injury, 3) must be applied before set is evident

* NAA/Sevin on >Empire=: 1) has been remarkably consistent from petal fall through 15 mm king fruit diameter (KFD), but usually yields better fruit size relative to cropload from earlier treatment; 2) petal fall applications often result in fruit size relative to cropload comparable to the best Accel/Sevin treatments; 3) usually breaks up clusters more effectively than Accel/Sevin

* in one instance, early application of NAA/Sevin did not result in as much return bloom as later applications despite similar thinning-results from numerous experiments suggest that this will be quite rare and should not pret growers from benefiting from greater fruit size of petal fall treatments

Use of Accel

* much more consistent response from Accel/Sevin combination than Accel alone, still more variable than NAA/Sevin

* no benefit to using Accel unless you get significant enhancement of fruit size

* in >Empire= best fruit size enhancement from Accel around 10 mm KFD, some response at 5-15 mm KFD but only thins from earlier treatments

* with >Empire=, in most cases, Accel/Sevin gives substantially greater crop value than NAA/Sevin

* use Accel at 35-75 ppm depending on experience with ease of thinning bock, Accel alone at 35-40 ppm has sporadically given excellent results with >McIntosh= where little thinning was needed

* excellent results in >Empire= from using Accel or Accel/Sevin at 10 mm KFD after earlier thinning treatment: results in similar fruit size advantage relative to cropload as Accel/Sevin alone

* we had poor results with Accel or Accel/Sevin in >Delicious= in 2 of 3 years o trials; other researchers (especially in Mid-Atlantic) report better response with lower rates than we have tried

* I suspect that use of Accel or Accel/Sevin at 10 mm KFD following cropload reduction with an earlier thinning treatment will enhance fruit size I many varieties but remember that using both NAA and Accel in the same growing can produce lot of pygmies in >Delicious= and >Fuji=.

Effect of environment conditions

* in two years of trials there has been no significant difference in thinning or fruit size from NAA/Sevin or Accel/Sevin when applied in days with high temperatures of 58-60^oF vs. 70-75^oF

* in one year, there was significantly more thinning when Accel was combined with oil, silwet, or Sevin as opposed to Regulaid or no adjuvant when rain followed within two years of application

* it appears that prolonged uptake under cool conditions that provide long drying may be as effective as more rapid but shorter uptake under warm >ideal= conditions

* there is considerable evidence that warm weather in the few days after application is the most important weather consideration for good thinning

Economics of apple thinning

* for most >Empire= blocks, greatest profitability (considering only the current season) is at croploads too high to ensure good return bloom - thus greatest long term profitability result from pushing cropping near but not up to the point of compromising return bloom

* to achieve croploads near the point of maximum economic return it will be necessary to keep records on cropload and returns within each block; and thin some blocks in stages, evaluating cropload before each subsequent treatment

* depending upon the block and the growing season, 40-75% 100 count fruit may yield the highest returns in >Empire= thinning to higher percent 100's, reduces number of 120s without increasing 100s

* for >Empire= in most blocks, use of Accel or Accel/Sevin will likely enhance profits

* in many >McIntosh= blocks, significant thinning reduces profit - in such blocks it is probably best to use either Sevin only, 3ppm NAA only to break up clusters, or 35-50 ppm Accel only to enhance fruit size

Pear thinning

* in two years of trials, Accel significantly increased fruit size and profitability in >Bartlett= compared to NAA. In the third year, only one Accel treatment increased fruit size, but set was low and fruit size was already very large

* Accel is not yet labeled for use on pears

* Endothall (an experimental blossom thinner) significantly thinned >Bartlett= in 1996 even though no other treatment was effective

Recommendations for apples

* obviously, your experience with each individual block is your best guide for developing thinning strategies

* consider using a blossom thinner on very difficult to thin or alternate bearing blocks - I believe these materials will be very useful on a limited acreage in New York

* prepare to apply your primary thinner (NAA/Sevin, NAA alone, or Sevin alone) as early as conditions permit following petal fall - make this application when conditions are good and warm weather is expected within 1-3 days

* when poor weather conditions prevent thinner application to all blocks under ideal conditions - if warm conditions are expected in the next 2-3 days, rather than delay application to large fruitlet size, it is probably better to make some applications when conditions are cool but provide good spray coverage

* in >Empire=, Accel/Sevin appears to be the best choice when a single thinning treatment will be used. In two trials we have had excellent results from thinning with NAA/Sevin at petal fall and then enhancing fruit size with Accel or Accel/Sevin at 10 mm KFD. This strategy is likely to be effective on many other varieties as well.

* review the comments on economics of apple thinning above. Many growers have probably developed thinning strategies that will provide greatest returns, but recognition of economic consequences and greater control over cropload could significantly enhance returns in many orchard blocks.

Cedar Apple Rust

Mark Your Calendar

The next orchard twilight meeting will be Tuesday, May 20, at Scott=s Fruit Farm, 1838 New London Tpke, Glastonbury, starting 5:30 PM.

_________________________________

David A. Kollas

Extension Pomologist

Phone: 860-486-1944

FAX: 860-486-0682